[CVE-2026-74679][MODERATE REGULAR] usb: gadget: f_ncm: Use unsigned int for ndp_index

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-74679
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: usb: gadget: f_ncm: Use unsigned int for ndp_index
Commit: 9c8c6825a750fcd3efbe922847ca70ccd5a66857
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9c8c6825a750fcd3efbe922847ca70ccd5a66857
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74679
Analysis date: Sat, 22 Aug 2026 12:12:52 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline manual review recommended
Manual review required: YES

Summary:
A signedness bug in USB gadget NCM parsing lets a malicious USB host supply a crafted NDP offset that bypasses bounds checks and causes an out-of-bounds read, leading primarily to kernel crash DoS and possible limited information exposure on gadget devices.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74679 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74679

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE REGULAR
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-74679	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H';CWE-190;CWE-681;CWE-125;*CWE-20;Other CVSS 'AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '6.1';DESCR 'A signedness bug in the USB gadget NCM receive path can make ndp_index wrap from a large unsigned offset into a negative signed value. The value is then used in mixed signed and unsigned bounds logic and can bypass sanity checks before the NDP block address is calculated from ntb_ptr plus ndp_index. A malicious USB host can trigger an out-of-bounds read by sending a crafted NCM NTB to a Linux device operating as a USB gadget. For the CVSS the PR:N because the malicious USB host does not need an account or privileges on the gadget device. The issue is not Internet reachable and normally requires physical USB access or control of the directly connected USB host. Impact is at least denial of service via kernel crash, with possible confidentiality impact from kernel out-of-bounds read. No clear write primitive is shown, so privilege escalation remains less supported than DoS or information exposure.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended (with actual score 3)	YES	OOB USB SIMPLEFIX IMPROVEONLY LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Weak or indirect corruption candidate: +1. The bug is an attacker-controlled signedness issue that can bypass bounds checks and produce an out-of-bounds read via `ntb_ptr + ndp_index`. This is not an OOB write or UAF.
* Reliable kernel crash / strong DoS: +1. The OOB read is in kernel USB gadget packet parsing and can plausibly crash the gadget system.
* Confidentiality impact plausible: +1. An OOB read may expose adjacent kernel memory depending on how parsing or error handling proceeds, but no explicit user-visible leak path is shown.
* Physical-only or rare hardware-only condition: -2. Triggering requires a malicious or controlled USB host connected to a Linux device running USB gadget NCM.
* Rare / non-default subsystem: -1. USB gadget NCM is not a broad default server path and is mostly relevant to embedded, device-mode, or specialized USB networking deployments.
* Availability impact realistic: +1. The malicious host can likely repeat crafted NCM traffic once physically connected, making DoS realistic for exposed gadget devices.

Paranoid adjustment:

* Paranoid score keeps the issue at 3 because the OOB read is attacker-controlled and in a protocol parser. It does not reach Actionable Moderate because there is no OOB write, UAF reclaim, callback control, type confusion, or privilege escalation primitive shown.

## 3. Reachability analysis

The attacker is a malicious USB host interacting with a Linux system configured as a USB gadget using the NCM function. No account or privileges are required on the gadget device itself, but the attacker needs physical USB connectivity or control over the directly attached USB host. This is not Internet reachable and not a common remote network path. Namespaces and containers are not the key factor here, because the vulnerable parser is reached from the USB gadget data path rather than a local namespace-controlled syscall interface. Path enablement depends on USB gadget NCM configuration, so exposure is deployment-specific.

Call-site confidence: medium. The supplied patch shows the vulnerable variable in `ncm_unwrap_ntb()` and the commit message clearly describes the downstream OOB read, but the full surrounding parser code is not included.

## 4. Severity interpretation

This behaves like a physical-adjacent memory-safety DoS with possible information exposure, not like an Important-class LPE. Theoretical memory corruption potential exists only as an out-of-bounds read. Realistic evidence supports crash and possible limited confidentiality impact, but not integrity impact or privilege escalation.

## 5. One-sentence report phrase

A signedness bug in USB gadget NCM parsing lets a malicious USB host supply a crafted NDP offset that bypasses bounds checks and causes an out-of-bounds read, leading primarily to kernel crash DoS and possible limited information exposure on gadget devices.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: attacker-controlled kernel OOB read should not be auto-closed without checking product exposure to USB gadget NCM, but the physical-only trigger and lack of write/UAF/LPE primitive keep it below Actionable Moderate.

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UPSTREAM PATCH SUMMARY
======================================================================

Patch: usb: gadget: f_ncm: Use unsigned int for ndp_index
Commit: 9c8c6825a750fcd3efbe922847ca70ccd5a66857
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9c8c6825a750fcd3efbe922847ca70ccd5a66857

Commit description:

The variable ndp_index is declared as a signed integer, but it stores
the return value of get_ncm(), which is unsigned.

A malicious host can supply a large offset that overflows the signed
ndp_index, making it negative. Because ndp_index is compared against
unsigned bounds, this negative value bypasses sanity checks and leads
to an out-of-bounds read when calculating the address of the NDP
block (ntb_ptr + ndp_index).

Fix this by changing ndp_index to unsigned int to ensure consistent
unsigned comparisons throughout the function.

Fixes: 370af73 ("usb: gadget: NCM: RX function support multiple NDPs")
Cc: stable <stable@kernel.org>
Signed-off-by: Sonali Pradhan <sonalipradhan@google.com>
Link: https://patch.msgid.link/20260720165654.2224591-1-sonalipradhan@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/usb/gadget/function/f_ncm.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9c8c6825a750fcd3efbe922847ca70ccd5a66857

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DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-74679 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74679

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS vector: AV:P/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
  The Best / paranoid CVSS score: 6.1

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 2
  Paranoid ActionableScore: 3

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).